Since the advent of the space age which began after the World War II era, the use of liquid fuel rocket engines has become commonplace. These engines often use cryogenic fuels, such as liquid oxygen and liquid hydrogen. Because of the very low temperatures of these fuels in the liquid state, insulated fuel tanks were needed. The application of sprayed-on foam insulation (SOFI) has proved to be the cheapest and best way to put the required insulation on the tanks. Sometimes, however, this foam insulation does not adhere well to the metal fuel tanks. Therefore, these debonded areas must be detected and repaired so that they will not cause problems connected with the firing and use of the high altitude rockets.
A second potential problem is possible weak spots in the sprayed-on foam insulation. This foam, which is a known type of polyurethane foam, is mixed as it is sprayed on. Sometimes, if the mixture is not proper, a reduction in the structural strength of the SOFI is experienced.
At the present time, there is a requirement for a test system to evaluate both the bonding and the strength of the insulation on the external tank of the Space Shuttle (or Space Transportation System, as it is known officially). The external tank (ET) is a tank which is attached to the Space Shuttle to furnish both liquid hydrogen and liquid oxygen to the main engines during launch. The external tank (ET) is completely covered with sprayed-on foam insulation. In addition, in the areas where the greatest amount of heat may develop during flight, such as the front end and the rear dome of the tank, a thin layer of an additional insulation called Super Light Ablator (SLA) is bonded to the tank. In these areas, SOFI is sprayed over the SLA insulation instead of directly on the surface of the ET.
In the recent past a test system called a "Porta Pull" has been used to perform a tensile test on the SOFI of the ET. This test consisted of pulling on a test plug cut in the insulation with an aluminum fitting bonded to the top of the test plug with an adhesive. "Porta Pull" consists of an air cylinder with a base supported on the insulation adjacent to the plug. Force readings are determined by the use of an air pressure regulator and pressure guage. The air cylinder area and the air pressure are used to calculate the tensile force applied to the plug. All testing is performed to failure, and when failure occurs the air cylinder "bounces" off the insulation surface. The "Porta Pull" system is cumbersome and requires the operator to perform calculations. Also the results are not to the desired accuracy.
Perhaps the closest prior art patent known to the applicant is U.S. Pat. No. 3,036,459 to KENDALL. The Kendall patent discloses a method of testing the strength of a reconstituted sponge. A helical spring is screwed into the surface of the sponge to the full length of the spring. A spring scale is then attached to the end of the spring and force is applied along the center line of the long axis of the spring to pull it directly out of the sponge. The force necessary to rupture the sponge and pull out the spring is read on the scale.
Several other prior art devices all have to do with testing the strength of concrete by pulling out a sample section of concrete with pulling devices having a piston pushed by hydraulic or gas pressure. Included in these devices are U.S. Pat. Nos. 3,541,845 to KIERKEGAARDHANSEN, 3,595,072 to RICHARDS, and 3,861,201 to KAINDL.
Still another type of prior art device is a device which performs tensile tests on a sample of material to determine its breaking point, which is then digitally displayed. Examples of this type of device are U.S. Pat. Nos. 3,862,902 to CLAXTON et al., and 4,235,114 to MOHLER.
Yet another type of prior art device is U.S. Pat. No. 2,835,128 to HERRSTRUM. This patent discloses an anchor pull device which measures the strength of a bolt anchor. The patent is concerned with bolt strength and not the strength of the surrounding material in which the bolt is embedded.
All the inventions disclosed in the above mentioned prior art patents appear to have at least some utility as tensile test devices. Most of these prior art patents concern the testing of a small specimen of a substance rather than testing the strength of a substance "in situ." Also, most of these patents relate to arrangements wherein a sample is placed between two jaws of the testing apparatus and one or both of the jaws are pulled away from the sample by any one of several well known power exerting devices, such as, for example, a motor, weights, or a hydraulic powered jack. However, none of the prior art patents disclose a device which could accomplish the purpose which is required of the instant invention. These prior art devices all appear to lack the versatility and complete portability, as well as the accuracy, convenience, and readability of the output, which is provided in the present system.
Therefore, the object of this invention is to provide an insulation test system which will check the strength and bonding of sprayed-on foam insulation on metal cryogenic fuel tanks. Another object is that the system including the power supply be lightweight and completely portable. A further object is that the system provide speed and accuracy without the need for calculations on the part of the operator.